442 
months represented also exhibit a plankton content above the 
average. The causes of this large production are to be found 
mainly in the prolonged low water, slackened current, and sew- 
age contamination of the last half of the year. No large ver- 
nal pulse appears in the records of the river or its backwaters. 
It was either intercalated between collections, and thus escaped 
detection, or the early winter flood, as in the previous year, by 
its washing away sources of nutrition prior to the season 
and temperature of greatest plankton development, tended to 
depress production below normal at this season. It was to be 
expected, however, that a large plankton development took 
place in the stable conditions attending the three months of 
declining levels which followed (Pl. XI.) the crest of the spring 
flood. If such a development took place it would tend to raise 
still higher the level of production established by our records 
for this year. 
The relation which the backwaters bear to channel produc- 
tion in 1897 iscorrelated with the hydrographic conditions. In 
January-June, a period of continued high water, the plankton 
content in the backwaters exceeds that in the channel in 22 of 
the 80 monthly averages (see table on p. 441), or, omitting Spoon 
River, which does not properly belong in the category of back- 
waters, in 21 out of 25. 
This was a period of extensive and long-continued impound- 
ing and of high levels, and, in the last three months (PI. XI.), 
of rapid decline and therefore of speedy run-off and rapid cur- 
rent in channel waters, factors which favor the breeding of the 
plankton in the reservoir regions and cut down the time for its 
development in the channel, in which barren tributary waters 
of recent origin and plankton-rich backwaters impounded in the 
more or less current-free areas for a greater or less length of 
time, depending upon the direction and rate of change in river 
levels, are mingled in varying proportions. 
In the low-water period, July-December, stability of hydro- 
graphic conditions continues throughout, while the extreme 
low levels maintained for so long a time make the channel wa- 
